Characterization of the sequence distribution and crystalline structure of poly(ethylene-co-vinyl acetate) copolymers with high-resolution NMR spectroscopy

The sequence distribution and crystalline structure of a series of poly(ethylene-co-vinyl acetate) (EVA) copolymers with different VA contents were investigated with high-resolution nuclear magnetic resonance spectroscopy (NMR). It was found that most of the VA segments are isolated in the main chain, though three kinds of sequence distributions (VA–VA head to tail, VA–E–VA head to tail, and VA–E–E–VA head to tail) exist in the EVA copolymers with higher VA content. Furthermore, two kinds of alkyl-branching signals were detected in the high temperature 13C NMR spectra of EVA copolymers. Solid-state 13C NMR spectroscopic investigation indicated that only orthorhombic phase exists in the crystalline region of the EVA copolymers with lower VA content. For the EVA copolymers with higher VA content, however, besides the occurrence of orthorhombic crystalline phase, monoclinic phase was also detected. As a metastable state, monoclinic phase is mainly affected by VA content as well as the thermal treating history.